1. Field of the Invention
The present invention relates to an endoscope system that produces not only a normal video image captured under normal light but also a special video image captured under special light, a processor device of the endoscope system, and an exposure control method.
2. Description Related to the Prior Art
An endoscope system is widely used in examination of the interior of a patient's body, e.g. a digestive system. The endoscope system is constituted of an electronic endoscope to be introduced into the patient's body, a light source device for supplying illumination light to the electronic endoscope, a processor device for processing an image (video image) captured by the electronic endoscope, and a monitor for displaying the image after being processed by the processor device. In recent years, there is known an endoscope system that carries out not only normal observation for imaging an internal body portion under white light (normal light) but also special observation for imaging the internal body portion irradiated with specific narrowband light (special light).
As the special observation, a blood vessel pattern obtaining technique is known in which a blood vessel in a specific depth is emphasized by applying the special light that has a wavelength having a high light absorption coefficient of hemoglobin to the internal body portion. Whether or not an obtained blood vessel pattern matches with a pattern specific to cancer is judged to find out a cancer-suspected lesion.
Also, there is known an oxygen saturation level obtaining technique. This technique uses first illumination light being narrowband light in a wavelength band at which a light absorption coefficient differs between oxygenated hemoglobin and deoxygenated hemoglobin, and second illumination light in a wavelength band different from that of the first illumination light. Sequentially applying the first and second illumination light to the internal body portion allows obtainment of an oxygen saturation level of blood. According to this technique, a hypoxic area being a cancer symptom is indicated by artificial color, so it is possible to find out cancer at sight.
To accurately find out cancer, a normal video image obtained in the normal observation and two types of special video images, that is, a vessel pattern video image and an oxygen saturation video image obtained in the special observation are preferably displayed together in a single monitor. Simultaneously displaying the plurality of types of video images, as described above, facilitates making a diagnosis from various viewpoints, and hence greatly improving diagnostic accuracy. Note that, Japanese Patent Laid-Open Publication No. 2003-033324 discloses simultaneous display of a plurality of types of video images in detail.
As described above, the normal video image, the vessel pattern video image, and the oxygen saturation video image are produced using the plurality of types of illumination light in the wavelength bands different from one another. Thus, in order to obtain and display the plurality of types of video images at the same time, lighting is sequentially switched to take out the illumination light required for producing each video image, and imaging is performed in synchronization with the switching of lighting. The amount of illumination light required for appropriate exposure differs from one video image to another, so it is necessary to perform exposure control in accordance with each video image. In general, the exposure control is performed based on the brightness of an immediately preceding frame image. However, in the case of switching and applying the plurality of types of illumination light in a sequential manner, the type of image to be subjected to the exposure control is different from the type of immediately preceding frame image. Therefore, the exposure control cannot be performed appropriately.